Optical disks, such as CD-ROM (Compact Disk-Read Only Memory), from which information can be read by semiconductor lasers and about 1.2 mm in thickness, have been available. For the reproduction of information from such an optical disk, a focusing servo control and a tracking servo control are performed on an objective lens and a pit string on a signal recording surface are irradiated with laser beams in an optical pickup device. Recently, higher densification has been achieved for recording animation lasting for a long time period.
For example, a DVD (Digital Video Disk) standard has been proposed. According to this standard, a DVD is formed as an optical disk recording information of 4.7 G bytes per one surface and having the same diameter with a CD-ROM, that is, 12 cm. By joining DVDs together back to back, each 0.6 mm in thickness, information of 9.4 G bytes can be recorded on one optical disk. Further, a double density CD that has a recording density twice the recording density of a CD and a CD-.alpha. that has a recording density three times the recording density of the CD are under development. Still further, an LD (Laser Disk) and a MUSE-LD (Multiple Sub-Nyquist Sampling Encoding-Laser Disk) recording a MUSE signal have been proposed.
Among these various optical disks, double density CDs, that are of the same diameter and thickness as CDs and hence attract attention as optical disks satisfying the requirement of high densification and requirements of players at the same time, are expected to be utilized as optical disks for karaoke or games, for example. If the double density CDs are to be employed as such, it is desirable to achieve high densification while maintaining the thickness of the double density CD same with CDs, that is 1.2 mm, to ensure the compatibility with the existing optical disks.
In the variety of currently available optical disks, various values are employed for a wavelength of a semiconductor laser employed in an optical pickup device, a numerical aperture of an objective lens, a track pitch and a minimum pit length of the recording medium. In the double density CD, the optimization of reproduction characteristic has not been considered in view of an actual setting of values such as the wavelength of the semiconductor laser, the numerical aperture of the objective lens, the track pitch, and the minimum pit length.
Hence, an object of the present invention is to provide a recording medium having favorable reproduction characteristics and an optical pickup device for the same.